In many instances it is desireable to mark items such as insulated conductors to provide information to persons installing or using such items. Such marking can often be done easily by color coding or by simple printing. As long as the material will accept ink, the only real limit on the amount of information which can be included is the available surface area. However, in the case of items produced from polytetrafluoroethylene (hereinafter "PTFE" throughout the remainder of the specification and the claims), such marking has been difficult due primarily to the physical and chemical attributes of the fluoropolymer. In some instances, colored pigments or dyes have been used, but this means by itself limits the amount of information that can be encoded. Furthermore, color coding may be of little or no use to those persons who are fully or partially color blind.
More recently, efforts have been made to incorporate into PTFE resins photosensitive fillers which react to subsequent laser radiation to produce the desired markings. One such recently developed marking method is described in detail in U.K. Patent 2,215,116. An earlier effort to provide satisfactory laser marking of such resins is described in European Patent Application 256,422. While each of these publications describes materials and processes which have some utility, each also has shortcomings. Thus, European Application 256,422 involves a two-layered insulation in which a portion of the outer layer is laser etched to permit a portion of the inner layer of differing color to become visible. Obviously, making a two-layered material is more difficult, time consuming and costly and, when the marking is done, portions of the outer layer have been physically degraded and are thus less useful as electrically insulating material.
U.K Patent 2,215,116 describes laser marking of a single layer insulation material having a photosensitive filler with minimal material degradation. However, these prior art materials have used filler materials of very small particle size and the results in practice have not been completely satisfactory due to insufficient stability of the color contrast after heat aging.